Preformed spas and baptismal pools normally include some means for heating the water. Typically the water in the pool is circulated by a pump through a small heater mounted adjacent one wall of the pool, the heater also including means to adjust the water temperature. Usually the heating means is electrical and includes a heating element mounted within a heat exchanger through which water from the pool flows. Over a period of use, especially with hard water, scale and sediment build-up on the interior of the heat exchanger surrounding the heating element, tending to insulate the heating element, to decrease its efficiency, and finally to cause it to overheat and burn out. So the chief aim of the present invention is to minimize as far as possible the build-up of such scale and sediment within heaters of the kind concerned and thus prolong the life of their heating elements.